The present invention concerns a method for oxidizing a surface region of a SiGe layer. It also concerns a method for treating a substrate comprising a semi-conducting layer on at least one of its surfaces, the method comprising an annealing of the substrate, the annealing itself comprising an oxidizing thermal treatment of the substrate under an oxidizing atmosphere for oxidizing a surface region of the substrate.
Such methods already exist in general. The annealing of substrates as it is exposed above is typically carried out in a controlled atmosphere. Annealing in a non-oxidizing atmosphere (nitrogen, argon, vacuum, . . . ) usually has the drawback of causing a phenomenon known as pitting on the surface of a semiconductor layer, particularly when it is silicon.
On the other hand, annealing in an oxidizing atmosphere is exposed to the drawback of creating defects in the crystal structure of the substrate.
U.S. Pat. No. 6,403,450 (US '450) proposes a solution for annealing a substrate without being exposed to a surface pitting, while at the same time limiting as far as possible the number of defects introduced into the crystal structure of the substrate. US '450 indeed proposes a method which comprises the creation of an oxidized region on a surface of the substrate, before the annealing of the substrate. In such method the oxidized surface region protects the substrate during the annealing—the annealing being carried out in an atmosphere which leaves the oxidized surface region unaffected. US '450 thus provides a solution for limiting the drawbacks mentioned above in relation with the annealing of a substrate.
This solution is well adapted, among others for the treatment of substrates of the SOI (Silicon On Insulator) type. However, the recent evolutions concerning the semiconductor substrates have promoted multilayer substrates which generally comprise a support layer in a material such as e.g. Si, and at least one layer in a semi-conducting material such as SiGe having a lattice parameter which is different from the lattice parameter of the support layer. SGOI (Silicon-Germanium On Insulator) substrates are an example of such substrates.
It would of course be interesting to apply the general solution provided by US '450 on a SGOI substrate. The general solution could be applied by creating the oxidized surface region through a deposition of an oxide. However, this would add a step of deposition in the process—which is not desired since such additional step would be associated to additional time, complexity and costs.
Another way of creating the oxidized surface region would be to proceed through a “direct” oxidation of the SiGe surface of a SGOI substrate (i.e. an oxidation of a surface region of the SiGe layer through a thermal treatment of the SiGe surface). But thermal treatments such as a thermal oxidation would be associated to undesirable effects within the SGOI substrate.
It is indeed known that annealing a SGOI substrate—e.g. for oxidizing its surface—can be associated to specific problems, and in particular to the generation of dislocations within the SiGe layer of the SGOI. Such dislocations within the SiGe layer are generated because of the formation, under the surface oxidized region of the SiGe layer, of a Ge-enriched layer. The annealing indeed rejects Ge from the surface region of the SiGe layer into the thickness of the SiGe layer, and this rejected Ge tends to pile up at the interface between the surface oxidized region and the underlying part of the SiGe layer. This pile up thus forms a Ge-enriched layer, buried between a surface oxidized region and the underlying remainder of the SiGe layer.
This Ge-enriched layer has a lattice parameter which is different from the lattice parameter of the underlying remainder of the SiGe layer. Therefore, as thermal oxidation of the SiGe layer is carried out and as the thickness of the Ge-enriched layer increases, this thickness would reach a value corresponding to the apparition of dislocations due to the lattice parameter mismatch between the Ge-enriched layer and the underlying SiGe. This limitation has been exposed in an article by LeGoues et al. (“Oxidation studies of SiGe”—J. Applied Physics 65(4), 15 Feb. 1989, 1724—see in particular part C: “Structural characterization”). There are thus limitations for applying the general method of US '450 to a SGOI substrate—in particular for applying it through direct oxidation. Accordingly, a new method is needed to avoid these limitations, and this is now provided by the present invention.